The present invention relates to a disconnecting circuit, and particularly to a solid-state device for blocking undesirable overvoltages and associated current transients from sensitive electronic circuitry.
In international patent application WO 95/01667, which is hereby incorporated by reference, a series protective circuit is described. The protective circuit includes a number of discrete high voltage field effect transistors (FETs) and is placed in series between a power source and a load to be protected. The protective circuit is configured so that under normal conditions the drain-to-source resistance of the FETs assumes a low resistance state. However, in response to a high voltage transient the protective circuit is configured to very rapidly open-circuit, thereby preventing damaging transmission of transient over-voltages to the load.
It will be realized that implementing the circuit by soldering or otherwise connecting the discrete FETs results in a bulky device, which is less desirable.
One approach to miniaturizing the device would be to fabricate the FETs of the circuit in silicon on the surface of a semiconductor wafer as shown in FIG. 2 (which shows three lateral JETs). While such an approach is well suited for implementing low voltage integrated circuits, it is not preferred where high power/voltage handling capability is desired. For example, circuits which must control AC motors or other inductive loads may need to operate at specified supply voltage magnitudes of 180V or more.
The present application describes a variety of transient blocking devices which use combined or merged transistor structures to achieve fast turn-off of a blocking transistor which can withstand high voltages. Preferably the blocking transistor is a depletion FET, e.g. a JFET or equivalent, in which the gate potential is driven by the body of a pass transistor.
Various embodiments describe various configurations for the blocking and pass transistors. In some classes of embodiments, the combination of a blocking transistor with two pass transistors is used for bidirectional transient blocking; in other embodiments only one pass transistor is used for unidirectional blocking.
Some embodiments use vertical pass transistors, which is more advantageous for low on-resistance and high blocking voltage; some others use lateral pass transistors, which is more advantageous for integrated embodiments. In such integrated embodiments a single solid structure, e.g. a high-voltage integrated circuit, combines transient blocking units as well as high-voltage circuitry.
Some embodiments use a gap between buried-layer diffusions to define the channel of the blocking transistor, which has the advantage of providing a high-breakdown-voltage device with good process integration capability.
Some embodiments use a gap between surface diffusions to define the channel of the blocking transistor, and place one source/drain terminal in an overlying (e.g. epitaxial) semiconductor layer. This configuration has the advantage of reduced susceptibility to latchup or problems caused by carrier diffusion.
Thus the present invention advantageously provides a solid-state circuit which is suitable for high voltage applications and which may be used to implement a transient protection circuit. Significant advantages of the innovative devices, systems, and methods include:    Rapid interruption of transient currents;    Ability to withstand voltages well above residential power line and subscriber line levels;    Device structure with capability for integration with power and/or smart power circuits;    Systems with very easy design specification;    Systems with reduced requirements for coordination of multiple types of protection devices;    Low on-state resistance;    Physically compact;    Good protection against grounding (earthing) problems, as well as against data or power line glitches;    Fast recovery from current interruption;    Autonomous recovery from current interruption, i.e. without requiring help from any external circuitry; and    Easy retrofit to existing system designs.
Further variations and optional features are described in the detailed description of exemplary embodiments which makes reference to a number of figures as follows.